(1) Field of the Invention
The present invention relates to a driving circuit of a pixel of an active display, and the driving circuit is capable of reducing the kink effect.
(2) Description of the Prior Art
An active matrix organic electroluminescent display (AMOLED) employs organic light emitting diodes (OLEDs) as light source, and thin film transistors (TFT) as switch or driver. The brightness of the organic light emitting diode is controlled by the current density. The current density of the organic light emitting diode is affected by the drain current of the thin film transistor, because the organic light emitting diode is usually connected to the drain electrode of the thin film transistor. However, the drain current is often influenced by the threshold voltage drift and the kink effect of the thin film transistor.
In an ideal case, the drain current (ID) is independent of the voltage (VDS) between the drain electrode and the source electrode. However, when the voltage (VDS) is larger than the pinched-off voltage, a depletion region is formed in the interface between the channel and the drain electrode so that the electrical distance between the drain and the source electrode, referred to as the “effective channel length”, is less than the physical channel length. When the differential voltage between the drain electrode and the source electrode is increased, the effective channel length is reduced. Because the effective channel length is inversely proportional to the drain current, as the differential voltage between the drain electrode and the source electrode is increases, so does the drain current. That is referred to as channel length modulation, or kink effect. The following illustrates that the influence of kink effect on the pixel.
FIG. 1A is a traditional driving circuit of a pixel of an active matrix organic electroluminescent display. The organic light emitting diode 101 has a cathode connected to a reference voltage generator VSS, and an anode connected to a drain electrode of a p-channel thin film transistor 102. The source electrode of the transistor 102 is connected to a display voltage generator VDD, and its gate electrode is connected to the gate electrode of another p-channel thin film transistor 103. The gate electrode and the drain electrode of the transistor 103 are connected to a drain electrode and a source electrode of a n-channel thin film transistor 105, respectively. The drain electrode of the transistor 103 is, moreover, connected to the drain electrode of another n-channel thin film transistor 106. The source electrode of the transistor 106 is connected to a data line 107. The transistors 105 and 106 act as switches, and their gate electrodes are connected to the scan line 108 and the data line 109, respectively.
When transistors 105 and 106 are opened, both transistors 102 and 103 act as a current mirror. The current IOLED flowing through the transistor 102 and the organic light emitting diode 101 is dependent on the current IDATA flowing through transistor 103. If the transistors 102 and 103 have the common property, the threshold voltage Vtp1 of the transistor 103 is equal to the threshold voltage Vtp2 of the transistor 102. The parameter μ pCox relating to their hole mobility is the same. The gate-source voltage VGS1 of the transistor 103 is equal to the gate-source voltage VGS2 of the transistor 102. Thus, the relationship is expressed as the equation (1):
                                          I            OLED                                I            DATA                          =                                            (                              W                /                L                            )                        2                                              (                              W                /                L                            )                        1                                              (        1        )            
Furthermore, if the channel length-width ratio of the transistor 102 is the same as that of the transistor 103, there is an ideal relationship expressed as IOLED=IDATA.
When the transistors 105 and 106 are opened, the equivalent circuit is shown as FIG. 1B. After opening the transistor 105, the gate electrode and the drain electrode of the transistor 103 are shorted, expresses as VDS1=VGS1.
Considering the influence of the kink effect, a factor λ is provided to multiply by the operating voltage VDS. If the transistor 102 and transistor 103 have the common property, such as the same μ pCox, Vtp1=Vtp2, VGS1=VGS2, and VDS1=VGS1, then IOLED and IDATA have the relationship expressed as the equation (2):
                                          I            OLED                                I            DATA                          =                                                            (                                  W                  /                  L                                )                            2                        ⁢                          (                              1                +                                  λ                  ⁢                                                                          ⁢                                      V                                          DS                      ⁢                                                                                          ⁢                      2                                                                                  )                                                                          (                                  W                  /                  L                                )                            1                        ⁢                          (                              1                +                                  λ                  ⁢                                                                          ⁢                                      V                                          GS                      ⁢                                                                                          ⁢                      1                                                                                  )                                                          (        2        )            
Even if the channel length-width ratio of the transistors 102 and 103 are the same, but VDS2≠VGS1, then IOLED≠IDATA.
When the channel length-width ratio W/L is 6/6 in the transistors 102 and 103, the result as FIG. 1C is obtained by simulating the equivalent circuit shown in FIG. 1B. The abscissa is time (sec), and the ordinate is current (A). The line 110 represents the current flowing through the transistor 103, associated with the current IDATA of the data line 107. The line 111 represent the current IOLED flowing through the organic light emitting diode 101. FIG. 1C shows that IOLED is different from IDATA in the current mirror, which is indeed affected by kink effect.
FIG. 1D is ID-VDS curve of a p-type metal oxide semiconductor (PMOS) including the low temperature poly silicon (LTPS). The value of W/L is shown as legend. In an ideal case, it should be horizontal at the right end of each curve, but in FIG. 1D, it turns upward. That illustrates the kink effect is possible to happen in PMOS so as to increase the drain current. Besides, as the PMOS has less physical channel length, its ID-VDS curve is more crooked. That represents it is affected by kink effect more apparently. Likewise, it also happens in an n-type metal oxide semiconductor (NMOS).
For reducing the kink effect, it needs to increase the voltage level of the display voltage generator VDD. As shown in FIG. 1D, take the curve of WIL=6/6 as an example, when the operating voltage VDS is larger than 2V, the transistors is operated in the saturation region of the ID-VDS curve. It is observed that they are affected by kink effect, because the slope of the curve is not zero between 2V and 4V. The slope of the curve is approach to zero between 4V and 6V in which the drain current of the transistors is controlled more easily. Therefore, it is necessary to rise the display voltage VDD a little, for example, to increase the operating voltage VDS from 2-4 V to 4-6 V. According to the prior art, IOLED is not still equal to IDATA even if rising the display voltage VDD.